Make integers come out in JSON as integers.

There are places (for example if you pass keepalive/threshold values to
the keepalive check in custom_client) where it matters about the type
of the variable coming from the JSON - Sensu looks for .is_a?(Integer)
and so if you quote the integers in the client config then it breaks.

In puppet everything is a string (except when it isn't - however you
cannot sanely predict when integers are going to be integers or strings)
and this it's necessary to make sure that anything which can become an
intger without harm is so, so that it's not quoted in the final JSON
we generate.